Ever since MacDiarmid, Hideki Shirakawa, and Heeger invented conducting polymers and made it possible to dope these polymers over the full range from insulator to metal, a new field of research bordering chemistry and condensed-matter physics emerged, which created a number of opportunities in the application in photoelectronic, electronic and electrochemistry. Conducting polymers have the advantages of stable physical and chemical properties and high conductivity. More significantly, conducting polymers provide an excellent interface between the electronic-transporting phase (electrode) and the ionic-transporting phase (electrolyte). In addition, the conductivity of conducting polymers is dependent on variables such as redox state and pH, which makes conducting polymers ideal for smart materials such as sensors.
In recent decades, conducting polymer hydrogels have received increasing attention for its promising applications in biosensors, chemical sensors, bioelectrodes, biobattery, microbial fuel cell, microbial electrolysis cell, medical electrodes, artificial muscle, artificial organ, drug release, and biofuel cells, etc. due to the following reasons:
1) Conducting polymer hydrogels have nanostructured framework and sufficiently large interfacial area, which enhanced the diffusion of ions and molecules, as well as the transport of electrons;
2) Conducting polymer hydrogels have a softer mechanical interface comparing to conventional metal electrodes;
3) Conducting polymer hydrogel have a biocompatible environment closely matching those of biological tissues.
To date, only a few limited methods were developed to synthesize conducting polymer hydrogels due to the difficulty in achieving the two prerequisite conditions for conducting polymers to form hydrogels: 1) hydrophilicity of polymer; 2) chemical or physical crosslinking between polymer chains. Synthesis of conducting polymer hydrogels has been carried out by following methods:
1) Synthesizing conducting polymer in the matrix of non-conducting polymer hydrogels (i.e. forming a composite material consisting of non-conductive hydrogel and conducting polymer);
2) Using multivalent metal ions such as Fe3+ or Mg2+ to crosslink water soluble conducting polymer such as poly(3,4-ethylenedioxythiophene) (PEDOT) by ions interacting with the negatively-charged electrolytic dopant;
3) Crosslinking polyaniline (PAni) by chemical reaction between epoxy group of the non-conducting crosslink agent and the amino group on PAni.
However, all of the above methods introduce impurities or nonfunctional materials, such as metal ions or nonfunctional polymers, thereby deteriorate the conductivity, electroactivity or biocompatibility of conducting polymers. In method 1), biocompatible composite material can be formulated using conducting polymer held in the matrix of hydrogels such as poly (vinyl alcohol), poly (ethylene glycol), and polyacrylamide chitosan, poly(2-hydroxyethyl methacrylate), poly (acrylic acid), poly (acrylamide), alginate hydrogel, etc. In this case, however, the non-functional polymer hydrogel impurity undoubtedly results in the lowering of conductivity and electroactivity of the material, which reduces the performance of electrodes and sensors. In method 2), crosslinked conducting polymer hydrogel is induced by ionic interaction of metal ions with negative polyelectrolyte dopant, which reduces the biocompatibility and enzyme activity of the hydrogels as high quantities of metal ions are required to form gels. In method 3), crosslinked PAni is made by the reaction between the epoxy crosslinking group and amino group on PAni main chain, which greatly reduces the conductivity of conducting polymers. In summary, the existing synthetic methods cannot meet the requirements of vast applications of conducting polymers, such as biomedical devices, biobattery, and microbial fuel cell.